The present disclosure relates generally to medical monitoring systems and, more particularly, to non-invasive medical monitoring systems employing optical sensors.
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present disclosure, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
A wide variety of devices have been developed for non-invasively monitoring physiological characteristics of patients. For example, a pulse oximetry sensor system may detect various patient blood flow characteristics, such as the blood-oxygen saturation of hemoglobin in arterial blood, the volume of individual blood pulsations supplying the tissue, and/or the rate of blood pulsations corresponding to each heart beat of a patient. To determine these physiological characteristics, light may be emitted into patient tissue, where the light may be scattered and/or absorbed in a manner dependent on such physiological characteristics.
Non-invasive medical sensor systems may include a medical sensor and an electronic patient monitor. The monitor may send driving signals to an emitter in the sensor, causing the sensor to emit light into pulsatile patient tissue. A detector in the medical sensor may detect the light after it has passed through the patient tissue, generating an electrical current proportional to the amount of detected light. This electrical current, referred to as a photocurrent, may be received by the patient monitor and converted into a voltage signal using a current-to-voltage (I-V) converter. The resulting voltage signal subsequently may be analyzed to determine certain physiological characteristics of the patient tissue.
When the I-V converter transforms the photocurrent from the photodetector to a voltage signal, thermal noise, also known as Johnson noise, may arise. The Johnson noise may be proportional to the square root of a transimpedance employed by the I-V converter, while the signal gain of the I-V converter may be directly proportional to the transimpedance. As a result, the higher the transimpedance, the higher the signal-to-noise ratio (SNR) of the I-V converter based on Johnson noise (e.g., when the transimpedance increases by a factor often, the SNR improves by a factor of √{square root over (10)}). On the other hand, the higher gain brought about by the higher transimpedance may cause the I-V converter to amplify the photocurrent beyond a signal saturation region of the I-V converter, which may produce a distorted output voltage signal.
The photocurrent and the emitter driving signals may share a cable between the medical sensor and the patient monitor. Despite cable shielding, capacitive and/or inductive coupling may occur between the emitter driving signals and the photocurrent. Thus, when the emitter driving signals rapidly change to turn the emitter on or off, a transient current may arise in the photocurrent. This transient current may discharge slowly as the I-V converter transforms the photocurrent to an output voltage signal. The higher the transimpedance of the I-V converter, the slower the I-V converter may discharge the transient current. Since the transient current of the photocurrent represents noise, the voltage signal that is obtained while the transient current is discharging may be noisy and therefore discarded. Accordingly, patient monitors may employ relatively low transimpedances to ensure the transient current discharges quickly enough to obtain a useful output voltage signal. However, a lower transimpedance may also provide a lower sensitivity.